Remote UV laser system and methods of use

ABSTRACT

A laser apparatus includes a modelocked laser system with a high reflector and an output coupler that define an oscillator cavity. An output beam is produced from the oscillator cavity. A gain medium and a modelocking device are positioned in the oscillator cavity. A diode pump source produces a pump beam that is incident on the gain medium. A second harmonic generator is coupled to the oscillator cavity. A third harmonic generator that produces a UV output beam, is coupled to the second harmonic generator. A photonic crystal fiber is provided with a proximal end coupled to the laser system. A delivery device is coupled to a distal portion of the photonic crystal fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/194,439, filed Jul. 12, 2002, titled Remote UV Laser System andMethods of Use, which is hereby incorporated by reference herein in itsentirety and which is a continuation-in-part of Ser. No. 10/114,337,filed Apr. 1, 2002, which is a continuation in part of Ser. No.09/321,499, filed May 27, 1999, now U.S. Pat. No. 6,373,565, issued Apr.16, 2002.

BACKGROUND

1. Field of the Invention

This invention relates generally to UV and visible laser systems, andtheir methods of use, and more particularly to UV and visible lasersystems that are suitable for semiconductor inspection or processing.

2. Description of Related Art

An increasing number of laser applications in the semiconductor industryrequire UV or visible laser light. These applications include inspectionas well as materials processing tasks. Many of these applicationsrequire that the sample under test be kept clean or be in closeproximity to processing equipment, and thus the entire machine islocated in a clean room environment.

Diode-pumped solid-state lasers are finding increasing acceptance inthis market because of their robustness. These systems consist ofseveral subsystems: a power supply to run the pump diodes, the pumpdiodes themselves, the laser head, and a harmonic conversion device togenerate the visible or UV radiation. Typically, the entire laser systemis included within the semiconductor-processing machine, which islocated in the clean room.

Diodes used as the pump source can be positioned in the power supply.Pump light is then coupled from the diodes in a multi-mode fiber, and isconveyed to the laser head by an armored fiber cable. In this way, thepower supply and diodes can be located remotely, while the laser headand harmonic conversion device are located in thesemiconductor-processing machine. The power supply and diodes can beoutside the machine or even outside the clean room.

However, positioning the diodes in the power supply, followed bycoupling the diode pump light in a multimode fiber, works because thepump light is: in the IR, continuous wave, and not diffraction limited.In contrast, the output of the laser is visible or UV, is often pulsed,and has a diffraction limited beam. Thus, single mode fibers arerequired to preserve the beam quality, but are problematic with bothpulses and UV radiation.

There is a need for improved UV and visible laser systems that aresuitable for semiconductor inspection or processing. There is a furtherneed for UV and visible laser systems for semiconductor inspection orprocessing applications where the laser resonator and power supply arepositioned at a location external to a clean room.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to providediode-pumped lasers, and their methods of use, in remote locationapplications.

Another object of the present invention is to provide diode-pumpedlasers, and their methods of use, in semiconductor inspection orprocessing applications with the laser resonator and power supplypositioned at a location external to a clean room.

These and other objects of the present invention are achieved in a laserapparatus that includes a modelocked laser system with a high reflectorand an output coupler that define an oscillator cavity. An output beamis produced from the oscillator cavity. A gain medium and a modelockingdevice are positioned in the oscillator cavity. A diode pump sourceproduces a pump beam that is incident on the gain medium. A secondharmonic generator is coupled to the oscillator cavity. A third harmonicgenerator that produces a UV output beam, is coupled to the secondharmonic generator. A photonic crystal fiber is provided with a proximalend coupled to the laser system. A delivery device is coupled to adistal portion of the photonic crystal fiber.

In another embodiment of the present invention, a laser apparatusincludes a modelocked laser system with a high reflector and an outputcoupler that define an oscillator cavity and produces an output beam. Again medium and a modelocking device are positioned in the oscillatorcavity. A diode pump source produces a pump beam that is incident on thegain medium. A first amplifier is also included. A second harmonicgenerator is coupled to the first amplifier. A third harmonic generatorthat produces a UV output beam, is coupled to the second harmonicgenerator. A photonic crystal fiber is provided with a proximal endcoupled to the laser system. A delivery device is coupled to a distalportion of the photonic crystal fiber.

In another embodiment of the present invention, a laser apparatusincludes a modelocked IR laser system with a high reflector and anoutput coupler that define an oscillator cavity. A gain medium and amodelocking device are positioned in the oscillator cavity. A diode pumpsource produces a pump beam that is incident on the gain medium. Aphotonic crystal fiber is provided with a proximal end coupled to the IRlaser system. A harmonic conversion delivery device is coupled to adistal end of the photonic crystal fiber.

In another embodiment of the present invention, a laser apparatusincludes a modelocked IR laser system with a high reflector and anoutput coupler that define an oscillator cavity. A gain medium and amodelocking device are positioned in the oscillator cavity. A diode pumpsource produces a pump beam incident on the gain medium. A firstamplifier is also included. A photonic crystal fiber has a proximal endcoupled to the IR laser system. A harmonic conversion delivery device iscoupled to a distal end of the photonic crystal fiber.

In another embodiment of the present invention, a method of delivering aUV output beam to a remote location provides a modelocked infrared lasersystem. The laser system includes a high reflector and an output couplerthat define an oscillator cavity that produces an output beam. A gainmedium and a modelocking device are positioned in the oscillator cavity.A photonic crystal fiber is provided and has a proximal portion coupledto the laser system, and a distal portion coupled to a delivery device.The infrared laser system is positioned at a distance from the remotelocation. A UV output beam is produced at a distance from the remotelocation. The UV output beam is delivered to the delivery device at theremote location.

In another embodiment of the present invention, a method of deliveringan UV output beam to a remote location is provided. A modelocked IRlaser system includes a high reflector and an output coupler that definean oscillator cavity that produces an output beam. A gain medium and amodelocking device are positioned in the oscillator cavity. A diode pumpsource produces a pump beam that is incident on the gain medium. Aharmonic conversion delivery device is positioned at the remotelocation. A photonic crystal fiber is provided that has a proximalportion coupled to the IR laser system, and a distal portion coupled tothe harmonic conversion delivery device. The IR laser beam is deliveredwith the photonic crystal fiber from the IR laser system to the harmonicconversion delivery device. A UV beam is produced from the harmonicconversion delivery device at the remote location.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram that illustrates one embodiment of a laser orlaser/amplifier system that produces UV light utilized with the systemsand methods of the present invention.

FIG. 2 is a block diagram of one embodiment of a system of the presentinvention illustrating the combination of the system of FIG. 1, aphotonic crystal fiber and a delivery device.

FIG. 3 is a block diagram that illustrates another embodiment of a laseror laser/amplifier system that produces IR light utilized with thesystems and methods of the present invention.

FIG. 4 is a block diagram of one embodiment of a system of the presentinvention illustrating the combination of the system of FIG. 3, aphotonic crystal fiber and a harmonic conversion delivery device.

FIG. 5 illustrates one embodiment of the present invention utilizing thesystems of FIG. 2 or FIG. 4 in a remote location.

DETAILED DESCRIPTION

In various embodiments, the present invention provides a laser apparatusthat has a laser system, and its methods of use. In one embodiment, thelaser system includes an oscillator system or an oscillator/amplifiersystem. The oscillator/amplifier system is similar to the oscillatorsystem but includes one or more amplifiers. The oscillator andoscillator/amplifier systems can be coupled with second, third, fourth,and fifth harmonic generators. A second harmonic generator can be usedalone with the oscillator and oscillator/amplifier systems and invarious combinations with third, fourth and fifth harmonic generators.Additionally, the harmonic generators can be coupled with an OPO. TheOPO can be pumped by a fundamental beam from an oscillator or from theharmonic generators. An output of the OPO can be mixed with the harmonicgenerators to generate an additional variable wavelength source.

In one embodiment, the oscillator system includes an Nd:YVO₄ gain mediumand is modelocked by a multiple quantum well absorber. In a specificembodiment of this oscillator system, the oscillator is pumped by asingle fiber-coupled diode bar that provides 13 watts of pump powerincident on the Nd:YVO₄ gain medium, and typically produces 5-6 watts of5-15 picosecond pulses at 80 MHz repetition rate. In another embodiment,an oscillator/amplifier system includes an Nd:YVO₄ gain mediummodelocked by a multiple quantum well absorber, a double pass amplifierand two single pass amplifiers. Each of the amplifiers has an Nd:YVO₄gain medium and is pumped by two fiber-coupled diode pump sources. Thisoscillator/amplifier system produces 25-30 watts of 5-15 picosecondpulses at 80 MHz repetition rate. In another embodiment, a pumpingwavelength of 880 nm is used for increased power with a similar value ofthe thermal lens in the gain medium.

The oscillator and oscillator/amplifier systems can be modelocked with amultiple quantum well saturable absorber, a non-linear mirrormodelocking method, a polarization coupled modelocking method, or othermodelocking techniques, including but not limited to use of an AOmodulator. An example of a quantum well saturable absorber is disclosedin U.S. Pat. No. 5,627,854, incorporated herein by reference. An exampleof a non-linear mirror modelocking method is disclosed in U.S. Pat. No.4,914,658, incorporated herein by reference. An example of apolarization coupled modelocking method is disclosed U.S. Pat. No.6,021,140, incorporated herein by reference. In order to produce shorterpulses and a single output beam the gain media is positioned adjacent toa fold mirror as described in U.S. Pat. No. 5,812,308, incorporatedherein by reference.

A high power oscillator system with the performance of anoscillator/amplifier system is achieved by using multiple fiber-coupleddiodes and either a non-linear mirror modelocking technique or apolarization coupled modelocking method. This high power oscillatorsystem produces 10-20 watts of output power with 4-10 picosecond pulsesat a repetition rate of 80-120 MHz.

High repetition rates are desirable for applications where the lasersystem is used as a quasi-CW source. For some applications, 80 MHzrepetition rate is sufficiency high to be considered quasi-CW. Thisrepetition rate is achieved with an oscillator cavity length of 1.8meters. When the cavity length is shortened to 0.4 meters the repetitionrate increases to 350 MHz.

Referring now to FIG. 1, one embodiment of an oscillator system 10 has aresonator cavity 12 defined by a high reflector 14 and an output coupler16. A gain media 18 is positioned in resonator cavity 12. Suitable gainmedia 18 include but are not limited to, Nd:YVO₄, Nd:YAG, Nd:YLF,Nd:Glass, Ti:sapphire, Cr:YAG, Cr:Forsterite, Yb:YAG, Yb:glass, Yb:KGW,Yb:KYW, KYbW, YbAG, and the like. A preferred gain media 18 is Nd:YVO₄.A modelocking device 19 is positioned in oscillator cavity 12. In oneembodiment, oscillator system 10 is modelocked and pumped by afiber-coupled bar 20 that produces 13 watts of power. Oscillator cavity12 can produce 1 to 6 watts of power nominally at an 80 MHz repetitionrate with pulse widths of 5 to 15 picoseconds.

Optionally included are one or more amplifiers, generally denoted as 23.An output beam 22 from resonator cavity 12 can be amplified by a firstamplifier 24. A second amplifier 26 can be included. Additionalamplifiers may also be included to increase power. Typically, amplifiers24 and 26 have the same gain media used in resonator cavity 12. Nd:YVO₄is a suitable gain media material because it provides high gain in anamplifier. The high gain of Nd:YVO₄ provides a simplified amplifierdesign requiring fewer passes through the gain media. Amplifiers 24 and26 produce output beams 28 and 30 respectively. Amplifiers 24 and 26 canbe single pass, double pass and four pass. A four pass amplifier isdisclosed in U.S. Pat. No. 5,812,308, incorporated herein by reference.Oscillator/amplifier system 10 using an oscillator, a double passamplifier and two single pass amplifiers can provide 30 watts of averagepower.

Output beams 22, 28 or 30 can be incident on a harmonic generatorgenerally denoted as 31 and can include a second harmonic generator 32.An output 34 from second harmonic generator 32 can be incident on athird harmonic generator 36 to produce an output beam 40. Alternatively,output 34 can be incident on a fourth harmonic generator 42 to producean output beam 44. It will be appreciated that oscillator system 10 caninclude various combinations of harmonic generators 32, 36, 42 as wellas a fifth or higher harmonic generators or an OPO. Second harmonicgenerator 32 can use non-critically phase matched LBO, third harmonicgenerator 36 can employ type II LBO and fourth harmonic generator 42 canuse type I BBO.

In a specific embodiment, oscillator system 10 includes oscillatorcavity 12 with harmonic generation. Output beam 22 is incident on secondharmonic generator 32. In this specific embodiment, oscillator system 10may also include third or fourth harmonic generators 36 and 42. Theoutput power of this oscillator system 10 is 5 watts at 1064 nm. Aharmonic generation system produces 2 watts at 532 nm or 1 watt at 355nm or 200 milliwatts at 266 nm.

In another specific embodiment, Nd:YVO₄ is the gain media ofoscillator/amplifier system 10, and 29 watts of 7 picosecond pulses at1064 nm is produced. The harmonic generation system can generate 22watts at 532 nm or 11 watts at 355 nm or 4.7 watts at 266 nm.

In another specific embodiment, oscillator/amplifier system 10 includesoscillator cavity 12, a four-pass amplifier 24 and second harmonicgenerator 32 to produce 2 watts at 532 nm. This oscillator/amplifiersystem can pump an OPO that utilizes non-critically phase matched LBO asdescribed in Kafka, et al., J. Opt. Soc. Am. B 12, 2147-2157 (1995)incorporated herein by reference.

In another specific embodiment, oscillator/amplifier system 10 includesoscillator cavity 12, a double pass amplifier 24 and three single passamplifiers 26 that produces 42 watts of 7 picosecond pulses at 1064 nm.This oscillator/amplifier system can pump an OPO using non-criticallyphase-matched KTA and produce an output beam at 1535 nm. The output beamat 1535 nm can be mixed with a 1064 nm beam to provide 11.6 watts at 629nm, as described in Nebel, et al., in Conference on Lasers andElectro-Optics, Vol. 6 of 1998 OSA Technical Digest Series (OpticalSociety of America, Washington, D.C., 1998) postdeadline paper CPD3.Fiber-coupled bars that produce 40 Watts, commercially available fromSpectra Physics Semiconductor Lasers, Tucson, Ariz. can be used toincrease the output power of oscillator or oscillator/amplifier systems10.

The use of a Nd:YVO₄ gain media 18 with a doping level of less than 0.5%can also be used to increase the output power of oscillator oroscillator/amplifier systems 10. The combination of the 40 wattfiber-coupled bars with the low doped Nd:YVO₄ gain media greatlyincreases the output power of oscillator and oscillator/amplifiersystems 10. Use of low doped Nd:YVO₄ gain media 18 can also reduce thesensitivity of oscillator cavity 12 to misalignment as well as improvethe output beam quality from an amplifier 24 or 26. The use of low dopedNd:YVO₄ gain media, a longer Nd:YVO₄ gain media as well as a larger pumpvolume in Nd:YVO₄ gain media is disclosed in U.S. Pat. No. 6,185,235,incorporated herein by reference. Oscillator system and/oroscillator/amplifier system 10, are collectively designated as lasersystem 110, and output beams 22, 28, 30, 34, 40 or 44 are collectivelydenoted as output beam 112.

Referring now to FIG. 2, one embodiment of the present invention is alaser apparatus 100 that includes laser system 110. A photonic crystalfiber 114 has a proximal portion 116 coupled to laser system 110 and adistal portion 118 coupled to a delivery device 120. Suitable deliverydevices include, but are not limited to, one or more lenses, mirrors,scanners, microscopes, telescopes, acousto-optic or electro-opticdevices, and the like.

A characteristic of photonic crystal fiber 114 is that is has lowabsorption at the wavelength of interest. Additionally, the damagethreshold and threshold for nonlinear effects are both high. By way ofillustration, and without limitation, the threshold for nonlineareffects can be substantially greater than 1 kilowatt. In one embodiment,photonic crystal fiber 114 is a hollow core single mode photonic crystalfiber. Hollow core single mode photonic crystal fiber 114 guides outputbeam 112 in air and preserves its mode quality. These fibers arecommercially available from Blaze Photonics, Bath, England.

As illustrated in FIG. 3, in another embodiment, laser system 210 is anIR laser system that produces an output of a wavelength between 1000 nmand 1100 and most preferably 1064 nm. The power range can be between 5to 30 W.

IR laser system 210 is similar to laser system 10 but does not includethe harmonic generators. IR laser system 210 has a resonator cavity 212,high reflector 214, output coupler 216, a gain media 218 and amodelocking device 219. IR laser system 210 is pumped by a pump source220 and produces an output beam 222. IR laser system 210 can include oneor more amplifiers, 223 that amplify output beam 222. Amplifier 223 caninclude a first amplifier 224, a second amplifier 226 and additionalamplifiers depending on the application.

Referring to FIG. 4, IR laser system 310 is similar to IR laser system210, and produces an output beam 312. Output beam 312 is coupled to aphotonic crystal fiber 314, which in turn is coupled to a harmonicconversion delivery device 320. Harmonic conversion delivery device 320can include various combinations of harmonic generators 332, 336, 342,as well as fifth or higher harmonic generators or an OPO, and a deliverydevice 338 which is substantially the same as delivery device 120.

In one method of the present invention, laser systems 110 or 310,collectively 410, are positioned remotely from a remote location 422.Delivery device 120 or harmonic conversion delivery device 320,collectively 420, is positioned at remote location 422. Output beams 112or 312, collectively 412, from laser 410, is delivered by photoniccrystal fiber 414 to delivery device 420 at a remote location 422 asshown in FIG. 5. In the embodiment of IR laser 310, its power supply,pump diodes, and IR laser head are all positioned away from remotelocation 422. Examples of remote location 422 include clean rooms,vacuum enclosures, enclosed machinery and the like.

In one embodiment, remote location 422 is a clean room that is utilizedin the semiconductor industry. However, it will be appreciated that thepresent invention also finds utility in a wide variety of differenttypes of clean rooms, and other remote locations, where it is desired toposition laser system 410 apart from remote location 422.

In one embodiment, laser system 410 is positioned from 2 to 200 metersfrom remote location 422. In another embodiment, laser system 410 ispositioned no more than 10 meters from remote location 422.

Laser system 410 is positioned away from remote location 422 and theheat produced by laser system 410 is not introduced to remote location422. By positioning laser system 410 away from remote location 422,maintenance of laser system 410 can be carried out without disruptingremote location 422 as well as items located there.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

1. A laser apparatus, comprising: a modelocked UV laser system includinga high reflector and an output coupler defining an oscillator cavity, again medium and a modelocking device positioned in the oscillatorcavity, a diode pump source producing a pump beam incident on the gainmedium, a second harmonic generator coupled to the oscillator cavity andto a third harmonic generator, the modelocked laser system producing aUV output beam; and a photonic crystal fiber with a proximal end coupledto the UV laser system.